A color liquid crystal display device is used to display an image, and this kind of device comprises two substrates, one that has a number of transparent scanning electrodes and another that has signal electrodes which face the scanning electrodes. A liquid crystal is provided between the two substrates, and a polarizing plate is provided on the outside of the two substrates. A color filter dyed in primary colors such as red, green and blue is provided with the above scanning electrodes. Further, the color filter is applied to an active matrix liquid crystal display wherein a thin film transistor is used as an active device. The crossing point of the scanning electrode and the signal electrode forms a pixel, and a adjacent three pixels constitute one component of a color image.
The color filter is formed on the transparent signal electrode which, in turn, is formed on the respective substrate. This kind of color filter is obtained by coating a colored base-material layer comprised of proteins such as casein or gelatine and then patterning the layer. The base material of the color filter is dielectric, and the liquid crystal is located between the color filter and the scanning electrode.
For this kind of color filter, a color pattern should be accurately formed for each color so that each color pixel is aligned with respective electrodes. Accordingly, a photolithography process should be performed after applying each colored, base-material layer.
In a method for manufacturing a conventional color filter, an align key pattern is formed on the substrate using chrome (Cr), and then a color resist layer is formed over the align key pattern. Then, using a laser beam and the align key pattern as a target, a mask is auto-aligned by moving the substrate. The color resist layer is then patterned by a conventional photolithography process.
FIG. 1 to FIG. 3 are cross-sectional views of the align key pattern regions used for forming red, green and blue resist patterns of the color filter according to a conventional method. Reference numeral 11 is a substrate, and reference numeral 12 is an align key pattern. Reference numeral 13 is a red resist layer, reference numeral 14 is a green resist layer, and reference numeral 15 is a blue resist layer. Reference numeral 16 is an anti-oxidative film, and reference numeral 17 is a laser beam. Each section of FIG. 1 to FIG. 3 has the same structure but for a different color layer 13-15. The conventional align key pattern and method for forming a color pattern using the same will be described below with reference to FIG. 1. It should be understood that the process will be similar for the other color layers of FIG. 2 and FIG. 3.
First, a chrome layer is deposited on substrate 11 to a thickness of approximately 1,500 .ANG., and then patterned using a conventional photolithography process to form align key pattern 12. Then, red resist layer 13 is formed, followed by anti-oxidative film 16. A mask for exposing red resist layer 13 is auto-aligned using a laser beam 17 whose wavelength is about 633 nm and align key pattern 12 as a target. Thereafter, red resist layer 13 is patterned via a conventional photolithography process, thereby forming a red color pattern.
Further, another mask is aligned using chrome, align key pattern 12 as a target, and then green and blue color patterns (FIG. 2 and FIG. 3) can be formed using the same manufacturing method as above. However, green and blue resists layers 14 and 15 have a very low (almost zero) transmittance of a 633 nm laser beam. FIG. 11 is a graphical representation showing the transmittance of a blue, green and red resists according to the wavelength of the laser beam. In FIG. 11, reference numeral 31 indicates the transmittance of a blue resist layer as a function of wavelength. Reference numeral 32 indicates the transmittance of a green resist layer, and reference numeral 33 indicates the transmittance of a red resist layer. When a green or blue color resist layer is formed, the light of the laser beam is not transmitted through the resist layer and cannot be sensed. The auto-aligner device has difficulty detecting the align key pattern. Auto-alignment is possible for the red color resist layer 13, whose transmittance of a 633 nm wavelength laser beam is above 90%, since the laser beam is reflected at the align key pattern and can be easily detected. However, the transmittance of the laser beam for green and blue resist layers 14 and 15 is almost zero for the above wavelength (much lower than that of the red resist). In addition to this, the chrome, align key pattern is thin and therefore, most of the remaining light is absorbed when the align key pattern is used. As a result, insufficient light is barely reflected from the align key pattern.
FIG. 4 is a plan view showing the align key pattern formed on the substrate and the align key pattern formed on a mask according to the conventional method. FIG. 5 shows the waveform diagrams detected from the light reflected from the align key pattern shown in FIG. 4.
In FIG. 4, a.sub.1, c.sub.1, a.sub.2 and c.sub.2 indicate the align key pattern formed on a substrate, and b.sub.1 and b.sub.2 indicate the align key pattern formed on the mask. As shown in FIG. 4, the laser beam reflected from align key pattern b.sub.1 and b.sub.2 on the mask is detected and aligned at the center of the align key patterns formed on the substrate. FIG. 5 shows waveform diagrams obtained from a group of align key patterns a.sub.1, b.sub.1, c.sub.1 and a.sub.2, b.sub.2, c.sub.2 of FIG. 4 using an oscilloscope when the mask is aligned. Waveform diagram (a) shows the case when a red resist layer is formed, and waveform diagram (b) shows the case when the blue or green resist is formed. As is shown in FIG. 5, it is difficult to perform an alignment process when the blue or green resist layer is formed, since the reflected wave is barely detected. Accordingly, the color resist layer over the align key pattern should be removed in order to perform auto-alignment of a mask. For this, an align key should be exposed manually by scrubbing the upper part of the key pattern using solvent (such as acetone or a developer) to remove a portion of the color resist layer over the align key pattern. However, this manual operation reduces productivity and yield.